500M-year fossil study shows coral-algae symbiosis advantage shifts with climate

Coral reefs have long served as a visible measure of planetary health, their bleaching and decline read as a warning sign of climate stress. But a new analysis spanning 500 million years of fossil evidence suggests the story is far more layered than the simple narrative of symbiosis as an unambiguous evolutionary win. An international team from Bristol, Wuhan, and Erlangen-Nuremberg universities has found that the advantage corals gain from hosting photosynthetic algae is not fixed in nature—it rises and falls entirely with the environmental conditions of the moment.

The research, published in the Proceedings of the National Academy of Sciences, divides the roughly 6,000 living coral species into two groups: those that depend on sunlight and the algae living inside their tissues, called Z corals, and those that thrive in deeper, darker waters without any algal partner, called AZ corals. Half of all coral species fall into each category, yet the deeper-water corals have received far less scientific attention. Led by Zhengsheng Wei, who conducted the work as a visiting student at Bristol under Professor Mike Benton, the team applied Bayesian modeling and artificial intelligence to vast fossil datasets, tracking how each group fared through major shifts in Earth's climate and chemistry over hundreds of millions of years.

What emerged was a striking reversal of fortune across geological time. During the Paleozoic era, the non-symbiotic AZ corals held the upper hand. The Z corals—the ones that would later become the iconic reef-builders—actually failed to recover after the Late Devonian extinction event. Both groups showed similar vulnerability to warming and oxygen-depleted waters. But when scleractinian corals arose in the Triassic period, the rules changed fundamentally. Photosymbiosis suddenly became the engine of diversification. Z corals gained a decisive edge, not because the partnership with algae is inherently superior, but because the environmental context had shifted in their favor.

The mechanisms driving success differed between the two groups in revealing ways. Z corals expanded through the creation of new species—origination—while AZ corals persisted mainly by avoiding extinction. This distinction matters. Z corals, living in shallow sunlit waters where they can harness their algal partners' energy, proved especially sensitive to rapid temperature swings. When waters warm, these corals expel their algae in a stress response, leading to the bleaching events now familiar from modern climate reporting. The deeper-dwelling AZ corals, by contrast, weather short-term temperature fluctuations more readily, their darker habitats offering some insulation from surface warming.

Yet this resilience has limits. As Mike Benton noted, while the deeper corals face less immediate risk from temperature spikes, they will eventually be forced to migrate away from waters that become persistently too warm, just as all marine animals must. The finding reshapes how scientists think about coral survival in a warming world. Conservation efforts have often treated corals as a monolithic group, but the fossil record reveals they are not. The shallow-water Z corals that build the vibrant reefs tourists visit and that support enormous biodiversity are acutely vulnerable to the temperature changes already underway. The deeper AZ corals, less visible and less studied, may prove more adaptable in the near term—though no coral can indefinitely outrun a fundamentally altered ocean.

Understanding how the rules of survival have shifted across deep time offers a lens for predicting what may come next. The study suggests that future conservation strategies cannot treat all corals the same way. Some populations may need active intervention; others may need protection of migration corridors as they seek cooler waters. The 500-million-year view does not offer comfort, but it does offer clarity: the partnership between coral and algae that seems so essential today was not always advantageous, and may not remain so. What matters now is recognizing which corals face which threats, and acting with that specificity in mind.